Sensors which make use of the well-known Hall effect have long been in widespread use, in particular for carrying out measurements on magnetic fields. In addition to the basic application of measuring the magnetic field strength, these sensors—referred to below as “Hall sensors”—are also used, for example, to measure the position, direction and rotational speed. A conceivable example of the latter application is the measurement of the rotational speed of gearwheels and driveshafts in machines. In addition, the Hall sensor serves, for example, to measure the change in the magnetic field strength when machine components consisting of a magnetic material, such as for example the teeth of a steel gearwheel, pass through the magnetic field of a permanent magnet. In this respect, it is also well known for the Hall sensors to be used in anti-lock brake systems (ABS) for vehicles. For many applications, it is essential for the permanent magnets/magnetic parts used to be as small and lightweight as possible. However, one significant drawback in this respect is the fact that Hall sensors according to the current state of the art are relatively unsuitable for the accurate measurement of weak magnetic fields (with a magnetic field strength of less than one milli-Tesla). This drawback generally has the effect of increasing costs. For example, for Hall sensors to be used effectively in the abovementioned ABS systems, it is necessary to use expensive rare earth magnets in order to create a sufficiently high magnetic field strength to allow sufficiently accurate measurements to be carried out with the aid of the sensor. Also, applications of this nature impose high demands on the linearity and accuracy of the required electronic amplification and signal-processing features and on the sensor housing, which likewise has the effect of increasing costs.
For many applications, it is desirable for Hall plates, generally together with amplification and signal-processing electronics, to be integrated in semiconductor material, for example in silicon using the well-known CMOS process. In addition to the advantage of simple integration of electronic components in silicon, this also has the disadvantage that the measurement accuracy and sensitivity of Hall plates according to the current state of the art are in fact adversely affected by factors which are inherent to their integration in silicon semiconductor material.
The traditional principle of carrying out measurements on magnetic fields using Hall plates, when using Hall plates integrated in silicon semiconductor material, also has an adverse effect on the measurement accuracy and sensitivity of the sensor. This is because in this traditional measurement principle, a current is used as excitation signal, and the resulting Hall voltage is measured, forming a representation of the field strength of the magnetic field in which the sensor is situated. One significant drawback of this is that mathematical analysis is able to demonstrate that an integrated Hall sensor has non-linearities which are dependent on an electric voltage and are very difficult to compensate for when using the abovementioned principle of current excitation and voltage detection.
The most important factors inherent in their integration in silicon semiconductor material which have an adverse effect on the measurement accuracy and sensitivity of Hall plates integrated in silicon semiconductor material in accordance with the current state of the art are:                offset voltages caused by mechanical stresses in the crystal lattices of the semiconductor materials used via the piezo-resistance effect;        offset voltages caused by the Seebeck effect: temperature differences create a position-dependent contact potential at the transition between semiconductor material and metal terminals at different locations on the Hall plate;        offset voltages caused by local geometric inaccuracies in the semiconductor material, formed during the integration process (for example alignment errors for the terminals, etching variations);        offset voltages resulting from accumulated charge at the transition between silicon and silicon oxide;        offset and non-linearities of electronic features for, for example, amplifying and processing output signals from Hall plates, the said circuits likewise being adversely affected by the abovementioned factors if they are integrated in semiconductor material, whether or not on the same substrate as the associated Hall plates themselves.        
Offset voltages in Hall sensors may be greater by a factor of 1000 than the Hall voltages which are ultimately to be measured. In the past, therefore, various methods have been developed attempting to compensate for the various offset voltages and other disadvantageous factors in order to increase the measurement accuracy and sensitivity of Hall sensors.
In a first approximation, a Hall plate can be modelled as a balanced resistance bridge (Wheatstone bridge). The abovementioned stresses in the crystal lattice of the semiconductor materials used changes the level of certain resistances in the bridge, resulting in the formation of an offset voltage which may be of the order of magnitude of a few tens of milli-Tesla. In addition, the abovementioned Seebeck effect is responsible for a static (current- and voltage-independent) offset voltage of the order of magnitude of a few milli-Tesla. This offset voltage is added to the output (Hall) voltage of the Hall plate. The Hall plate then delivers an output voltage where no magnetic field is present. The magnetic field strength which would have to be measured with an “ideal” Hall plate in order to generate a Hall voltage of the same order of magnitude as this offset voltage may easily amount to several tens of milli-Tesla.
By mathematical analysis, it is possible to demonstrate that the abovementioned static offset resulting from the Seebeck effect can be compensated by carrying out measurements in pairs, with the direction of the excitation current being reversed for the second sub-measurement in each case and the difference in the Hall voltage resulting from the two sub-measurements then being determined.
Mathematical analysis can also be used to demonstrate that the abovementioned offset resulting from stresses can be compensated for by carrying out measurements using two Hall plates, with the second Hall plate rotated through 90° with respect to the first. The difference in the output (Hall) voltages from the two Hall plates is in each case determined. U.S. Pat. No. 5,241,270 uses this method in modified form, with two Hall plates employed simultaneously, so that the two measurements mentioned above can be carried out simultaneously, rather than in succession.
Numerous known methods which attempt to compensate for the offset resulting from stresses are based on a configuration also known as an “orthogonally switched Hall plate”, since the current directions of the excitation currents are perpendicular to one another in the two sub-measurements. Most Hall sensors according to the current state of the art comprise a square Hall plate with electrical terminals at the corners. In the case of the abovementioned offset compensation method using orthogonally switched Hall plates, the measurements are in most cases carried out in pairs, in which case in the first sub-measurement an excitation current is passed through the Hall plate between two opposite terminals, and the resulting Hall voltage is measured across the two other, opposite terminals. Instead of reversing the direction of the excitation current as described above, for the second sub-measurement the pairs of terminals for the excitation current and the Hall voltage are swapped over, so that the direction of the excitation current is now rotated through 90° with respect to the direction in the first sub-measurement. The polarity of the Hall voltage which is measured during the second sub-measurement is then inverted, and this voltage is added to the measured Hall voltage from the first sub-measurement. Inter alia, patent documents U.S. Pat. No. 5,406,202, U.S. Pat. No. 5,844,427, EP 1 010 987 A2 and EP 1 130 360 A2 describe Hall sensors in which offset compensation methods of this type with orthogonally switched Hall plates are used. This method can only provide complete offset compensation if the Hall plates used were to have a completely linear behaviour in functional respects. On account of their design, however, Hall plates formed in semiconductor material are inherently nonlinear. It can be demonstrated that the most important nonlinearities in Hall plates are dependent on an electric voltage. However, since the offset compensation methods described above use current excitation and voltage detection, it is impossible to completely compensate for nonlinear offset terms. Moreover, according to the method described of orthogonal switching of Hall plates, the direction of the excitation current cannot be completely (180°) reversed, but rather can only be turned through 90°, and consequently the offset resulting from the Seebeck effect is not compensated for, with the result that a significant offset term remains present. The literature has disclosed offset compensation methods which make use of the abovementioned orthogonal switching, but through 360° rather than through 90°. Hall sensors in which this method is used are known in the literature as spinning current Hall sensors. The Hall plates used in this case are generally provided with eight terminals and have a symmetry which is such that in each case a straight connecting line between two opposite terminals is orthogonal (perpendicular) with respect to a straight connecting line between two other terminals. In this case, during eight sub-measurements, in each case a fixed excitation current passes between two opposite terminals, and the associated Hall voltage is measured between the two terminals whose straight connecting line is orthogonal with respect to the straight connecting line between the two terminals mentioned first. The resulting Hall sensor is produced by the German Fraunhofer IIS. The relatively large number of terminals of the Hall plate in this sensor, however, is responsible for an undesirable reduction in the sensitivity of the sensor with respect to Hall plates having a smaller number of terminals. In this case too, current excitation and voltage detection are used, and consequently nonlinear offset terms are not fully compensated for.
U.S. Pat. No. 5,621,319 describes a method for compensating for the offset resulting from mechanical crystal stresses in integrated Hall sensors. Use is made of the above-described spinning method with orthogonal switching of the Hall plate. In addition, use is made of voltage excitation rather than current excitation. However, the drawback is that this voltage excitation is combined with voltage detection, and consequently the offset resulting from stresses is not in fact compensated for, on account of the directional-dependent nature of electrical properties of the semiconductor material (anisotropy).
In many patent documents, such as the abovementioned U.S. Pat. No. 5,406,202 and U.S. Pat. No. 5,844,427, which describe methods for compensating for the offset resulting from crystal stresses in integrated Hall sensors, it is attempted to achieve initial offset compensation by parallel-connection of a plurality of Hall plates which are rotated through a defined angle with respect to one another. In most cases, this involves two Hall plates which are rotated through an angle of 90° with respect to one another. It can be demonstrated by mathematical means that this approach can only function optimally if four Hall plates are connected in parallel, of which the second, third and fourth plates are respectively rotated through 90, 180 and 270° with respect to the first plate, and if voltage excitation and current detection are also used. Document EP 1 206 707 B1 does indeed use a configuration with four Hall plates, but these plates are only rotated through in each case 45° rather than 90°. In functional terms, the four Hall plates in this case in reality form a single spinning current Hall plate with eight terminals, as described above, with the associated drawbacks as likewise described above.
A further significant source of offset in Hall sensors is the offset and nonlinearity of electronic features for, for example, amplifying and processing output signals from Hall plates. The fact that these electronic features are often integrated with the Hall plates in the same semiconductor substrate offers possibilities for, for example, combining compensation for the offset of an integrated amplifier with compensation for the offset of a Hall plate resulting from the Seebeck effect. U.S. Pat. No. 6,154,027 describes a method in which the output signal of a spinning current Hall plate is firstly pre-amplified before being demodulated. However, this involves spinning through 90° in two stages rather than spinning through 360° in four stages. Consequently, the offset resulting from the SEEBECK effect is not compensated for.
The offset resulting from mechanical crystal stresses in the semiconductor material of a Hall sensor varies for different crystal directions. Nevertheless, the relevant literature in this field provides scarcely any information about the optimum orientation of a Hall plate in semiconductor material. Research carried out by the Applicant has demonstrated that the sensitivity of a Hall plate to stress can be reduced by a factor of 10 by selecting the appropriate orientation.
To summarize, on the basis of what has been stated above, it can be concluded that hitherto it has not been possible to solve the problems inherent to the current state of the art in this field in order to sufficiently compensate for the effect of factors which have a negative influence on the measurement accuracy and sensitivity of integrated Hall sensors.
It is an object of the present invention to eliminate the abovementioned drawbacks.